The process industry uses a variety of devices to monitor and control industrial processes. Such devices include process variable transmitters, process actuators, process alarm devices, and process control modules.
Process variable transmitters are used to monitor process variables associated with industrial processes. Such variables include pressure, temperature, flow, level, pH, turbidity, density, concentration, chemical composition and other properties. Process actuators are used to control valves, pumps, heaters, agitators, solenoids, vents, and other such devices. Process alarm devices are used to remotely monitor a specific process variable provided by a process variable transmitter, among other things, and provide an alarm if the process variable deviates unacceptably from the process norm. Process control modules receive information related to the process from process variable transmitters, perform analyses upon the received process information and initiate corrective action through process actuators. Generally, a process control module is located in a control room to facilitate user interaction.
Due to the volatile nature of the process field environment, process devices should not generate a spark that could couple to and ignite explosive atmospheres and flammable material. Process devices generally satisfy this design criteria by either having explosion-proof housings as specified in NEC Sections 500-503, dated 1996, or by having intrinsically safe circuits. When a process device is intrinsically safe, it operates on such low power levels that it is generally not able to generate a spark with enough energy to cause ignition even under fault conditions. This design criteria is further complicated by the fact that additional external power sources are often not available to power a process device in the field. Thus, the process device must rely upon power supplied from the process loop itself while being either explosion-proof or intrinsically safe. Low power process devices are able to meet intrinsic safety criteria while still operating solely upon power received from a process control loop.
In the process industry, rapid and reliable communication between the process devices is very important. In the past, such communication involved a process variable transmitter controlling the amount of current flowing through a process control loop based upon a process variable. Current was supplied from a current source in the control room and the process variable transmitter controlled the amount of current from its location in the field. For example, a 4 milliamp (mA) signal could be used to indicate a zero reading while a 20 mA could be used to indicate a full-scale reading. As technology progresses, there is an ever-increasing demand to provide more and more information about the process and the process devices themselves.
Foundation(trademark) Fieldbus and Profibus-PA (referred to collectively as fieldbus) are multi-drop serial digital communications protocols intended for connecting field instruments and other process devices such as monitoring and simulation units in process control systems. The physical layer of the fieldbus protocols are defined by Instrument Society of America standard ISA-S50.02-1992, and its draft 2 extension dated 1995; or IEC 1158-2 dated 1993. Fieldbus allows enhanced digital communication over previous process control loop methods while maintaining the ability to power process devices coupled to the fieldbus loop and while meeting intrinsic safety requirements.
The introduction of fieldbus initially provided a digital process loop capable of allowing communication at 31,250 bits/second. Later, revisions of fieldbus allow 1 megabit/second and 2.5 megabits/second rates of digital serial communication. Other communications rates are also contemplated.
Fieldbus now provides significant capabilities for digitally communicating immense amounts of process data. Thus, there is a continuing need to develop process devices capable of maximizing fieldbus communication effectiveness while minimizing power consumption, cost, and device size.
A method and apparatus are provided to allow at least portions of two fieldbus messages to be stored in a fieldbus device. The fieldbus device includes a media access unit, a fieldbus communication controller, and a controller. The media access unit is coupleable to a fieldbus loop to receive fieldbus signals and provide a digital bitstream related to the fieldbus signals. The fieldbus communication controller assembles data segments relating to at least portions of two fieldbus messages from the bitstream and stores the segments in a receive FIFO memory. The controller is adapted to read the segments from the receive FIFO memory and act upon fieldbus messages.